In the drilling of wells, such as oil or gas wells, core samples of porous rock are taken of the earth formation through which the wells are drilled and various characteristics of the rock are determined for the purpose of establishing different fluids in the formation, estimating the quantity of each fluid in the formation, the ease of flow through the formation, etc. Such core samples are also taken from producing reservoirs and characteristics of the rock are determined for the purpose of estimating particular fluid quantities, predicting production rates, etc. Particularly important among the characteristics of the porous rock is determining oil and water saturation and the porosity and permeability of the reservoir.
U.S. Pat. No. 4,920,792 teaches the use of liquified gas extraction to extract oil and brine fluids from a representative porous sample, however, it does not teach determining fluid saturation at an overburden stress which approximates reservoir stress conditions, which is especially important for unconsolidated samples.
It is a specific object of the present invention to provide a new liquified gas extraction method for determining the oil and water saturation of porous core samples at overburden pressure and net stresses representative of those in the reservoir. This new method can be used with consolidated and unconsolidated rock or fractured rock in which pressure-cycling can be a problem. This new method provides separate determination of oil and water or brine saturations, and does not alter rock wettability and causes less damage to clay minerals and gypsum. It is another object of this invention to provide a method that can be used for determining current oil in place and porosity at reservoir pressure without cycling the confining pressure on the formation rock sample. This new method can be readily modified for the determination of solution gas composition, gas-oil ratio (GOR), and oil density and viscosity at reservoir conditions.